1. Field of the Invention
The present invention relates in general to direct access storage devices and more particularly, to a method and apparatus for providing compensation for shifts in recording levels in a direct access storage device which utilizes a fixed-delay tree search detector, where such shifts are due to changes in magneto-resistive (MR) head temperatures and contact of the MR head with asperities.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is recorded on concentric tracks on either surface of one or more magnetic recording disks. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and to move the heads. The read/write heads must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
To accurately write and read data, it is desirable to maintain the head on the center of the track. To assist in controlling the position of the head, each sector of the disk typically contains a number of servo bits accurately located relative to the centerline of the track. The signals produced by the servo bits are typically demodulated into position offset signals which are used to determine the position of the head relative to the track, and to move the actuator arm if the head is not located on the track centerline.
There has been developed a dual element transducer which includes a single write element and a separate read element that is constructed from a magneto-resistive material. Such dual element transducers are commonly referred to as magneto-resistive (MR) heads. Due to its sensitivity, an MR head is particularly susceptible to errors caused by thermal asperities, which are recording signal anomalies caused by contact between an MR head and asperities on the media surface. Asperities may be caused by manufacturing defects, dust or flecks of metal oxide on recording media surfaces. Thermal asperity related errors may be caused by rapidly rising MR head temperatures due to momentary contact with an asperity. A rise in temperature may change head resistance, causing a corresponding transient in output signal voltage. Since MR resistance varies with variation in the amplitude of the magnetic field, transients in MR head voltage correlate directly to errors. When severe, thermal asperity-induced errors may be unrecoverable because the number of resulting errors may exceed the syndrome or the capability of error correcting codes (ECC) used in read processing circuits.
One particularly troublesome problem with thermal asperities, which make them difficult to detect and correct, is their timing. Thermal related transients in MR head outputs may occur with rise times measured in nanoseconds. Fast rise times may make thermal asperities difficult to detect in a sufficient amount of time to invoke corrective measures. Moreover, with durations of several microseconds, thermal asperities may cause continuous error bursts before dissipating. In addition, the amplitude of transients caused by thermal asperities may be greater than twice the peak amplitude of the MR head read signal.
One approach to this problem is described by Galbraith et al. in "Magneto-Resistive Head Thermal Asperity Digital Compensation", which discloses an analog to digital converter expanded headroom technique and a timing and gain correction-hold technique. This approach, however, has the disadvantage of reduced signal-to-noise ratio when operated in an expanded headroom mode. Moreover, lack of control of the thermal asperities during the hold periods when timing and control loop operation is suspended may cause a complete loss of read data synchronization and gain synchronization.
A second approach to this problem is disclosed in U.S. Pat. No. 5,367,409 to Ottensen, which discloses even harmonic distortion compensation for digital data detection. The Ottensen technique requires the use of an analog-to-digital converter (ADC) for providing a sampled readback signal and an adder for providing a compensated output signal. The compensated output signal is then applied to the digital channel. The Ottensen technique, is however, complex and the hardware costs associated with the ADC and the adder make the practical implementation of the technique difficult.
U.S. patent application Ser. No. 08/634,189, entitled "Thermal Asperity and Baseline Shift Compensation" which was assigned to the assignee of the present invention, describes a method and apparatus for removing transient DC level shifts caused by thermal asperities in a direct access storage device. This application describes the removal of such transient DC level shifts in a direct access storage device which utilizes a PR4 Viterbi Detector.
As the density at which data is written on a magnetic recording channel increases relative to the width of the isolated transition response of the channel, intersymbol interference (ISI) imposes a limit on the error-rate that can be achieved using standard bit-by-bit detection methods like peak detection. There has been developed a Fixed-Delay Tree Search (FDTS) Detector, which when used in conjunction with an appropriate encoding and/or decoding scheme, would provide a read signal with an improvement of several decibels over conventional PR4 or PR5 Viterbi detectors. However, to date, there has yet been developed a method for providing compensation for shifts in recording levels in a direct access storage device which utilizes a FDTS detector, where such shifts are due to changes in magneto-resistive (MR) head temperatures and due to contact of the MR head with asperities.
Accordingly, there is a need in the technology for a simple, effective and elegant method and apparatus for providing compensation for shifts in recording levels in a direct access storage device which utilizes a FDTS detector, where such shifts are due to changes in magneto-resistive (MR) head temperatures and due to contact of the MR head with asperities.